1. Field of the Invention
The present invention relates to a small rotary electrical machine in which a current flowing through a coil is not used for a field magnet weakening control. An extensive operation range covering a high torque low speed rotation through a low torque high speed rotation is thus provided, and a control thereof is easily done with high efficiency.
2. Description of the Related Art
Conventionally, radial gap type electric motors, such as a radial gap type rotary electrical machine, are used as a drive source for electromotive two-wheeled vehicles or the like. Such radial gap type electric motors, and other general electric motors, have a structure in which a yoke of a rotor (rotor side yoke) and a yoke of a stator (stator side yoke) oppose each other, and in which their opposing surfaces extend parallel to an axis of a rotating shaft supported by bearings.
The opposing surface of the rotor side yoke has a plurality of field magnets circumferentially disposed on a cylindrical inner wall, while the opposing surface of the stator side yoke has a plurality of teeth radially disposed thereon so as to oppose the surface of the rotor side yoke. A coil is wound around each one of the plurality of teeth. That is, in the radial gap type electric motor, the opposing surfaces of the field magnets and the teeth extend about axes transverse to the axis of the rotating shaft, and the opposing surfaces define an annular gap therebetween about the rotating shaft. That is, the gap in the radial gap type electric motor is defined in a direction generally transverse to the axis of rotation.
In contrast, although being a kind of the radial gap type electric motor, there is another electric motor that has a configuration in which the stator side yoke is cylindrically formed, and the rotor side yoke is columnar and positionable within the cylinder. One such type of electric motor is proposed, for example, in Japanese Publication No. JP 2000-261988. Such an electric motor has a cylindrical member in which a permeable section and an impermeable section are alternately positioned between opposing surfaces of ends of respective projections of a stator core of the stator core side yoke and opposing surfaces of respective permanent magnets of the rotor side yoke to prevent cogging from occurring and also to bring in a low torque operation in a high speed rotation.
Another electric motor in which the stator side yoke is cylindrically formed and the rotor side yoke is columnar to be positioned within the cylinder, is proposed, for example, in Japanese Publication No. JP 2004-166369. In order to reduce the stator flux linkage in the high speed rotation, the stator core of the stator side yoke is constructed with a cylindrical core extending about an axis of rotation and a bar-like core reciprocating within the cylindrically shaped core in a direction generally transverse to the axis of rotation. The bar-like shaped core moves in the transverse direction of the stator core relative to a coil that is circumferentially wound around the cylindrically shaped core.
Recently, in addition to the radial gap type rotary electrical machines, axial gap type rotary electrical machines have attracted a great deal of attention. For example, an axial gap type electric motor, as one of the axial gap type rotary electrical machines, has a disk-like rotor side yoke including a rotational shaft supported by its bearings and a disk-like stator side yoke with a center aligned with an axis of the rotational shaft. The disk-like rotor side yoke and disk-like stator side yoke are disposed opposite each other.
On a surface of the rotor side yoke, a plurality of field magnets are circularly (or annularly) disposed along a disk-like circumferential portion thereof. Likewise, a plurality of teeth is disposed along a disk-like circumferential portion of a surface of the stator side yoke. The surface of the rotor-side yoke and the surface of the stator-side yoke are disposed opposite each other. Also, the opposing surfaces of the field magnets and of the teeth define a gap therebetween, and the opposing surfaces define a surface that crosses the rotational shaft at right angles (i.e., perpendicularly crosses the rotational shaft). That is, the gap is formed to extend in a direction along the rotational shaft, i.e., axially.
One method to vary an output characteristic of an axial gap type electric motor, as thus described, includes moving either a rotor (the rotor side yoke having field magnets) or a stator disposed to oppose to the rotor (a coiled core positioned on the stator side yoke) in a direction of the rotational shaft to control a distance between the rotor and the stator. Therefore, an amount of magnetic flux flowing between the field magnets and the coiled core is controlled. However, though conventional gap type electric motors can vary the output characteristic by increasing the gap between the rotor and the stator, they necessarily require that the electric motor units be bulkier to allow for said variation in gap size. Such increased bulkiness is contrary to the desire to have gap type electric motors (both axial gap type and radial gap type) be as small as possible.